1. Field of Applicable Technology
The present invention relates to a magnetic recording medium which incorporates a strongly magnetic metallic thin film, and in particular to a magnetic metallic thin film type of recording medium which has high durability and reliability. The invention is applicable to a magnetic recording medium in the form of a recording disc or recording tape.
The invention is specifically directed towards a metallic thin film type of recording medium which is formed of a plurality of successive layers having protective and lubricative functions in addition to the metallic thin film, i.e. a film of carbon which is in the diamond state, having formed thereon a protective film of carbon, and with an uppermost layer of a lubricative material. A metallic thin film type of recording medium has the basic advantage of a high level of output signal upon playback, and so provides a high level of signal/noise (S/N) ratio. However, in the prior art, it has been a basic disadvantage of such a metallic thin film type of recording medium that it is difficult to obtain a sufficient degree of durability. In the case of a metallic thin film recording tape, this durability can be measured as the limit of the duration for which the tape can be continuously operated, e.g. in a VTR (video tape recorder) when the VTR is operating in the "still" playback mode. In such a mode, a single diagonal region of the recording tape is repetitively scanned by the recording heads of the VTR, resulting in wear occurring in that region of the tape. The time duration for which such "still" operation can be continued before deterioration of the playback image obtained from the recording tape begins to be visibly perceptible (that duration being referred to in the following simply as the "still" mode operating limit of the tape) can be used as a measure of the durability of the recording tape, and such a measure will be used in the following description. In the case of a magnetic recording disc, the measure of durability is generally the "contact start/stop" capability, or the CSS figure, which is a measured as a maximum number of passes of the magnetic head with respect to the recording disc which can occur before damage to the disc begins.
2. Prior Art Technology
Various techniques have been proposed in the prior art for improving a magnetic recording medium of the type which employs a strongly magnetic metallic thin film as a magnetic recording layer. Such proposals have been directed towards improving the durability of such a magnetic recording medium, i.e. to improving the "still" mode operating limit, in the case of a recording tape, and to improving the CSS figure, in the case of a recording disc. One prior art proposal has been to provide a lubricative layer formed of carboxylic acid, over the metallic thin film. Another proposal has been to form a protective thin film of a non-magnetic material such as SiO.sub.2 over the metallic thin film. Yet another proposal has been to form a protective thin film of carbon or of diamond-state carbon over the metallic thin film. However with such prior art methods, in which a lubricative layer alone, or a protective film alone is utilized, insufficient durability is obtained for the case of a recording tape, as indicated by the "still" mode operating limit, particularly when the recording tape is operated under high tension. Similarly, insufficient durability is obtained for a magnetic recording disc manufactured by such a method, i.e. the CSS figure is insufficient.